Soft tissue tumors of the extremity, benign and malignant, occur in people of all ages. The incidence of these tumors will likely increase in the coming years due both to an aging population and to the longer survival of patients with a previous cancer treated with radiation and/or chemotherapy, both known to be risk factors for sarcoma. The gold standard for treatment of soft tissue tumors is complete surgical resection of the tumor and in the case of malignant tumors an additional margin of healthy tissue. Despite advances in surgical techniques, imaging technologies, chemotherapy regimens, and radiotherapy neither the morbidity associated with treating these tumors nor the survival for patients with malignant tumors has significantly improved over the past 20 years. Accordingly, there is a need for novel treatment modalities to improve the outcomes for patients who have soft tissue tumors both in terms of survival and of treatment-related morbidity. Magnetic Resonance guided High Intensity Focused Ultrasound (MRgHIFU) ablation is a noninvasive treatment modality that is currently in clinical use for treating uterine fibroids and in investigational use to treat prostate, breast, and other types of cancer. We will adapt MRgHIFU techniques to the treatment of benign and malignant soft tissue tumors of the extremities with the goal of safely and effectively performing HIFU ablation on human subjects. A series of cadaver experiments will define the best methods to couple the ultrasound transducer to the extremity, determine the accuracy of MRgHIFU treatment in different tissues such as tendon, fat, muscle and near bone, and characterize the effect of using different ultrasound frequencies on the ablation of target tissue and the surrounding healthy tissue. In an animal model, we will determine how closely MRgHIFU ablation can be performed relative to important anatomic structures such as major peripheral nerves without causing injury. We will test the accuracy and effectiveness of the HIFU treatment in patients prior to standard of care surgical removal of the tumors. Also, we will optimize the post ablation imaging techniques to assess effectiveness of treatment.